[Flux plugin] Add namespace selector

[mo-saeed]

Right now we create flux resources in different namespace for multi-tenancy so we have flux resources in multiple namespace.
All views has namespace selector/filter where i can see resources for each namespace. would be great to add the same for Flux plugin.

[joaquimrocha]

@ashu8912 I think this is useful, right?

[mgalesloot]

See PR headlamp-k8s/plugins#110 for a first attempt to solve this.